Synthesis and Characterization of the Oxynitride Y2Mo2O4.5N2.5 Pyrochlore: A Neutron Diffraction and Magnetic Study

Abstract

Abstract A new molybdenum oxynitride Y2Mo2O4.5N2.5 with cubic pyrochlore structure (a = 10.3350(2) Å, space group Fd3̅m, Z =8) has been synthesized by heating the parent Y2Mo2O7 oxide in flowing ammonia at 898 K. The polycrystalline sample has been characterized by thermal analysis, X-ray and neutron diffraction (NPD), and magnetic susceptibility measurements. The analysis of high resolution NPD data, based on the contrast existing between the scattering lengths of O and N, shows that both atoms are distributed at random at the anion substructure; the refined crystallographic formula implies an oxidation state of +5.25 for Mo cations. The thermogravimetric curve shows a weight gain of 7.5% at 1000 K in air, corresponding to the complete elimination of N2 and oxidation to Mo(VI) oxide, in good agreement with the proposed composition. The magnetic susceptibility exhibits a Pauli-like, temperature-independent term which derives from the partial delocalization of Mo electrons on Mo-(O,N) bands with a broader bandwidth, as a consequence of the significant opening of the Mo-(O,N)-Mo angle and strengthening of the Mo-(O,N) interactions with respect to the parent Y2Mo2O7 oxide. As in this oxide, a reminiscent spin-glass behaviour is observed at low temperature.